1. Field of the Invention
This invention relates generally to photovoltaic solar cells and to a method of making the cells. More particularly, the invention relates to the improvement in stability of a copper chalcogenide/cadmium chalcogenide thin-film photovoltaic cell.
2. The Prior Art
Cadmium sulfide solar cells operate on the principle of converting light energy to electrical energy. These cells comprise a thin film of cadmium sulfide deposited on a bottom electrode. On one surface of the cadmium sulfide a film of copper sulfide is placed which forms a barrier junction between the films. Without going into an explanation of how it is generated, the incidence of light on the copper sulfide layer, or as some maintain, on the cadmium sulfide, effects a voltage and current between the cadmium sulfide layer and the copper sulfide layer. In order to collect this current from the cell, a "collector grid" is applied to the copper sulfide layer to form the top electrode. By connecting appropriate wires to the top and bottom electrodes, a circuit can be formed for the flow of electrical current.
Stoichiometry of the copper sulfide layer is considered an important factor in providing highly efficient cells. Efforts are made to provide a copper sulfide layer of chalcocite (Cu.sub.2 S) and to avoid other less efficient forms such as djurleite (Cu.sub.1.95 S) and digenite (CU.sub.1.70 S). For example, in U.S. Pat. No. 3,887,697 to Bogus et al and in U.S. Pat. No. 4,167,805 to Castel, a copper sulfide layer is deposited on the cadmium sulfide film and is brought to the desired stoichiometry by copper diffused from metallic copper in contact therewith.
Although efficient thin film cadmium sulfide photovoltaic cells can be formed, one of the major problems associated with such cells is the lack of stability in prolonged use. Air and moisture are common sources of degradation. In some other cases, such as when the top and bottom electrodes come into contact causing an electrical short, the source of the degradation is obvious, and means to correct the situation are also obvious. In other cases, though, the source of the degradation is not readily known. The present invention deals with a solution to such a degradation situation not previously understood.